I'm looking to brew a saison next. I have a 300 watt aquarium heater but was wondering how if it will melt the rubbermaid plastic or better bottle for that matter if its to close. How does everyone secure their heaters.

While there is water in there the temperature will not get high enough to melt plastic. The water will take the heat away. If you start seeing bubbles (boiling water) locally from around the heater, you might want to be concerned about what is going on, but that shouldn't happen.

I've used an aquarium heater on a plastic tote before, but I was only trying to warm from about 60F ambient air temp to 65F. Note that using just the aquarium heater, didn't warm up the entire bath. You need some motion to the water or you will only be heating the water right around the heater. I used a stir plate attached to the outside of my tote and the stir bar in the tote underwater to get the motion that kept the entire bath constant.

Here's a picture of my setup (it's pretty ghetto, but it worked - once I added in the stir plate). Note that I also added bricks to the one on the left lower the amount of water needed. Next time I will add in some bleach as the water got kind of nasty after a week like that.